Cold conditioning is the process of holding finished ale at low temperatures (close to freezing) for several days. Fermentation activity ceases and the yeast drops out of suspension, leaving a clear beer with a smooth flavour. In effect, it gracefully ages the beer, leaving it much more stable over time.
Both lagering and conditioning are best accomplished in a fridge dedicated to beer. External thermostats can easily convert an old fridge or chest freezer to a beer cooler. Remember the temperature not only needs to be cold, it must also be steady. Fluctuating temperatures can make the yeast produce off flavours, spoiling the beer.